GLFW: DPI scaling seems to be applied twice on style on Wayland

[Cyphall]

Version/Branch of Dear ImGui:

Version 1.92.8, Branch: docking

Back-ends:

imgui_impl_glfw.cpp + custom render backend

Compiler, OS:

Linux Fedora 44

Full config/build information:

Using GLFW X11 backend:

Dear ImGui 1.92.8 (19280)
--------------------------------

sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=201703
define: IMGUI_DISABLE_OBSOLETE_FUNCTIONS
define: __linux__
define: __GNUC__=4
define: __clang_version__=22.1.6 (Fedora 22.1.6-1.fc44)
define: IMGUI_HAS_VIEWPORT
define: IMGUI_HAS_DOCK
IM_ASSERT: runs expression: OK. expand size: OK
--------------------------------

io.BackendPlatformName: imgui_impl_glfw (3400)
io.BackendRendererName: ImGuiVulkanBackend
io.ConfigFlags: 0x00000080
 DockingEnable
io.ConfigDpiScaleFonts
io.ConfigDpiScaleViewports
io.ConfigViewportsNoDecoration
io.ConfigViewportsNoDefaultParent
io.ConfigNavCaptureKeyboard
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x0000181E
 HasMouseCursors
 HasSetMousePos
 PlatformHasViewports
 HasMouseHoveredViewport
 RendererHasVtxOffset
 RendererHasTextures
--------------------------------

io.Fonts: 1 fonts, Flags: 0x00000000, TexSize: 512,256
io.Fonts->FontLoaderName: FreeType (2.14.3; 2.14.3)
io.DisplaySize: 3840.00,2006.00
io.DisplayFramebufferScale: 1.00,1.00
--------------------------------

style.WindowPadding: 16.00,16.00
style.WindowBorderSize: 2.00
style.FramePadding: 10.00,8.00
style.FrameRounding: 4.00
style.FrameBorderSize: 2.00
style.ItemSpacing: 16.00,8.00
style.ItemInnerSpacing: 8.00,8.00

Using GLFW Wayland backend:

Dear ImGui 1.92.8 (19280)
--------------------------------

sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=201703
define: IMGUI_DISABLE_OBSOLETE_FUNCTIONS
define: __linux__
define: __GNUC__=4
define: __clang_version__=22.1.6 (Fedora 22.1.6-1.fc44)
define: IMGUI_HAS_VIEWPORT
define: IMGUI_HAS_DOCK
IM_ASSERT: runs expression: OK. expand size: OK
--------------------------------

io.BackendPlatformName: imgui_impl_glfw (3400)
io.BackendRendererName: ImGuiVulkanBackend
io.ConfigFlags: 0x00000080
 DockingEnable
io.ConfigDpiScaleFonts
io.ConfigDpiScaleViewports
io.ConfigViewportsNoDecoration
io.ConfigViewportsNoDefaultParent
io.ConfigNavCaptureKeyboard
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x0000101E
 HasMouseCursors
 HasSetMousePos
 HasMouseHoveredViewport
 RendererHasVtxOffset
 RendererHasTextures
--------------------------------

io.Fonts: 1 fonts, Flags: 0x00000000, TexSize: 512,256
io.Fonts->FontLoaderName: FreeType (2.14.3; 2.14.3)
io.DisplaySize: 1829.00,955.00
io.DisplayFramebufferScale: 2.10,2.10
--------------------------------

style.WindowPadding: 16.00,16.00
style.WindowBorderSize: 2.00
style.FramePadding: 10.00,8.00
style.FrameRounding: 4.00
style.FrameBorderSize: 2.00
style.ItemSpacing: 16.00,8.00
style.ItemInnerSpacing: 8.00,8.00

Details:

When running with GLFW's Wayland backend, DPI scaling is applied twice on styles (fonts seem to be unaffected).

My app does this to handle DPI:

ImGuiIO& io = ImGui::GetIO();
io.ConfigDpiScaleFonts = true;
io.ConfigDpiScaleViewports = true;

float pixelScale;
glfwGetWindowContentScale(window, &pixelScale, nullptr);

ImGuiStyle& style = ImGui::GetStyle();
style.ScaleAllSizes(pixelScale);
style.FontSizeBase = 14;

(full source code here)

Also, looking at the build infos, io.DisplaySize and io.DisplayFramebufferScale are different between X11 and Wayland.

Screenshots/Video:

Using GLFW X11 backend:

Using GLFW Wayland backend:

Minimal, Complete and Verifiable Example code:

Put one of those lines as the first line of the main() function in example_glfw_opengl3 to compare backend behaviors:

glfwInitHint(GLFW_PLATFORM, GLFW_PLATFORM_X11);
// or
glfwInitHint(GLFW_PLATFORM, GLFW_PLATFORM_WAYLAND);

The symptoms are slightly different here, Wayland shows a blurry render instead of double-scaled styles, as if the scaling was done by the compositor rather than the app itself.

The difference comes from the source of the DPI scale passed to ImGui::ScaleAllSizes:

• The example gets it from ImGui_ImplGlfw_GetContentScaleForMonitor, which always returns 1 on Wayland for some reason (X11 correctly reports 2.1).
• My app gets it from glfwGetWindowContentScale, which correctly reports 2.1 on both X11 and Wayland.